Aromatic carbonate is important as a raw material for the production of an aromatic polycarbonate, which is the most widely used engineering plastics, without using toxic phosgene. As a process for producing an aromatic carbonate, a process of reacting an aromatic monohydroxy compound with phosgene has been known from long ago, and has also been the subject of a variety of studies in recent years. However, this process has the problem of using phosgene, and in addition chlorinated impurities that are difficult to separate out are present in the aromatic carbonate produced using this process, and hence the aromatic carbonate cannot be used in an application in which high purity is required, for example as a raw material for the production of an aromatic polycarbonate.
On the other hand, it is also known that a process for producing aromatic carbonates through transesterification reactions between a dialkyl carbonate and an aromatic monohydroxy compound. However, since such: transesterification reactions are all equilibrium reactions and the equilibrium is biased extremely toward the original system, and the reaction rate is slow, and there have been many difficulties in producing aromatic carbonates industrially in large amounts using this method. In addition to development of catalysts being carried out, many attempts have thus been made to devise a reaction system so as to shift the equilibrium toward the product system as much as possible, and thus improve the aromatic carbonate yield. For example, for the reaction between dimethyl carbonate and phenol, there have been proposed a method in which by-produced methanol is distilled off by azeotropy together with an azeotrope-forming agent (see Patent Document 1: Japanese Patent Application Laid-Open No. 5448732 (corresponding to West German Patent Application No. 736063, and U.S. Pat. No. 4,252,737)), and a method in which the by-produced methanol is removed by being adsorbed onto a molecular sieve (see Patent Document 2: Japanese Patent Application Laid-Open No. 58-185536 (corresponding to U.S. Pat. No. 410,464)). Moreover, a method has also been proposed in which, using an apparatus in which a distillation column is provided on top of a reactor, an alcohol by-produced in the reaction is separated off from the reaction mixture, and at the same time unreacted starting material that evaporates is separated off by distillation (see Patent Document 3-1: examples in Japanese Patent Application Laid-Open No. 56-123948 (corresponding to U.S. Pat. No. 4,182,726); Patent Document 3-2: examples in Japanese Patent Application Laid-Open No. 56-25138; Patent Document 3-3: examples in Japanese Patent Application Laid-Open No. 60-169444 (corresponding to U.S. Pat. No. 4,554,110); Patent Document 3-4: examples in Japanese Patent Application Laid-Open No. 60-169445 (corresponding to U.S. Pat. No. 4,552,704); Patent Document 3-5 examples in Japanese Patent Application Laid-Open No. 60-173016 (corresponding to U.S. Pat. No. 4,609,501); Patent Document 3-6: examples in Japanese Patent Application Laid-Open No. 61-172852; Patent Document 3-7: examples in Japanese Patent Application Laid-Open No. 61-291545; Patent Document 3-8: examples in Japanese Patent Application Laid-Open No. 62-277345).
However, these reaction systems have basically been batch system or switchover system. This is because the extent to which the reaction rate can be improved through catalyst development is limited for such a transesterification reaction, and hence the reaction rate is still slow, and thus it has been thought that a batch system is preferable to a continuous system. Of these, a continuous stirring tank reactor (CSTR) system in which a distillation column is provided on top of a reactor has been proposed as a continuous system, but there are problems such as the reaction rate being slow, and the gas-liquid interface in the reactor being small based on the volume of the liquid. It is thus not possible to make the conversion high. Accordingly, it is difficult to attain the object of producing an aromatic carbonate continuously in large amounts stably for a prolonged period of time by means of the above methods, and many issues remain to be resolved before economical industrial implementation is possible.
The present inventors have developed reactive distillation methods in which such a transesterification reaction is carried out in a continuous multi-stage distillation column simultaneously with separation by distillation, and have been the first in the world to disclose that such a reactive distillation system is useful for such a transesterification reaction, for example, a reactive distillation method in which a dialkyl carbonate and an aromatic hydroxy compound are continuously fed into the multi-stage distillation column, and the reaction is carried out continuously inside the column in which a catalyst is present, while continuously withdrawing a low boiling point component containing an alcohol produced as a by-product by distillation and continuously withdrawing a component containing a produced alkyl aryl carbonate from a lower portion of the column (see Patent Document 4: Japanese Patent Application Laid-Open No. 3-291257), a reactive distillation method in which an alkyl aryl carbonate is continuously fed into the multi-stage distillation column, and the reaction is carried out continuously inside the column in which a catalyst is present, while continuously withdrawing a low boiling point component containing a dialkyl carbonate produced as a by-product by distillation, and continuously withdrawing a component containing a produced diaryl carbonate from a lower portion of the column (see Patent document 5: Japanese Patent Application Laid-Open No. 4-9358), a reactive distillation method in which these reactions are carried out using two continuous multi-stage distillation columns, and hence a diaryl carbonate is produced continuously while efficiently recycling a dialkyl carbonate produced as a by-product (see Patent document 6: Japanese Patent Application Laid-Open No. 4-211038), and a reactive distillation method in which a dialkyl carbonate and an aromatic hydroxy compound or the like are continuously fed into the multi-stage distillation column, and a liquid that flows down through the column is withdrawn from a side outlet provided at an intermediate stage and/or a lowermost stage of the distillation column, and is introduced into a reactor provided outside the distillation column so as to bring about reaction, and is then introduced back through a circulating inlet provided at a stage above the stage where the outlet is provided, whereby reaction is carried out in both the reactor and the distillation column (see Patent Documents 7-1: Japanese Patent Application Laid-Open No. 4-224547; Patent Document 7-2: Japanese Patent Application Laid-Open No. 4-230242; Patent Document 7-3: Japanese Patent Application Laid-Open No. 4-235951).
These reactive distillation methods proposed by the present inventors are the first to enable aromatic carbonates to be produced continuously and efficiently, and many similar reactive distillation systems based on the above disclosures have been proposed thereafter (see Patent Document 8: Italian Patent No. 01255746; Patent Document 9: Japanese Patent Application Laid-Open No. 6-9506 (corresponding to European Patent No. 0560159, and U.S. Pat. No. 5,282,965); Patent Document 10: Japanese Patent Application Laid-Open No. 6-41022 (corresponding to European Patent No. 0572870, and U.S. Pat. No. 5,362,901); Patent Documents 11: Japanese Patent Application Laid-Open No. 6-157424 (corresponding to European Patent No. 0582931, and U.S. Pat. No. 5,334,742); Patent Document 12: Japanese Patent Application Laid-Open No. 6-184058 (corresponding to European Patent No. 0582930, and U.S. Pat. No. 5,344,954); Patent Document 13: Japanese Patent Application Laid-Open No. 7-304713; Patent Document 14: Japanese Patent Application Laid-Open No. 9-40616; Patent Document 15: Japanese Patent Application Laid-Open No. 9-59225; Patent Document 16: Japanese Patent Application Laid-Open No. 9-110805; Patent Document 17: Japanese Patent Application Laid-Open No. 9-165357; Patent Document 18: Japanese Patent Application Laid-Open No. 9-173819; Patent Documents 19-1: Japanese Patent Application Laid-Open No. 9-176094; Patent Document 19-2: Japanese Patent Application Laid-Open No. 2000-191596; Patent Document 19-3: Japanese Patent Application Laid-Open No. 2000-191597; Patent Documents 20: Japanese Patent Application Laid-Open No. 9-194436 (corresponding to European Patent No. 0785184, and U.S. Pat. No. 5,705,673); Patent Documents 21: International Publication No. 00/18720 (corresponding to U.S. Pat. No. 6,093,842); Patent Document 22-1: Japanese Patent Application Laid-Open No. 2001-64234; Patent Document 22-2: Japanese Patent Application Laid-Open No. 2001-64235; Patent Document 23: International Publication No. 02/40439 (corresponding to U.S. Pat. Nos. 6,596,894, 6,596,895, and 6,600,061)).
Among the reactive distillation systems, the present applicants have further proposed, a method carried out while keeping the weight ratio of a polyhydric aromatic hydroxy compound in the reaction system to a catalyst metal at not more than 2.0 (see Patent Document 24: International Publication No. 97/11049 (corresponding to European Patent No. 0855384,. and U.S. Pat. No. 5,872,275)), as a method that enables highly pure aromatic carbonates to be produced stably for a prolonged period of time without a large amount of a catalyst being required, a method in which a high boiling point material containing a catalyst component is reacted with an active substance and then separated off, and the catalyst component is recycled (see Patent Document 25: Japanese Patent Application Laid-Open No. 11-92429 (corresponding to European Patent No. 1016648, and U.S. Pat. No. 6,262,210)). Furthermore, the present inventors have also proposed a method in which 70 to 99% by weight of phenol produced as a by-product in a polymerization process is used as a starting material, and diphenyl carbonate can be produced by means of the reactive distillation method. This diphenyl carbonate can be used as the raw material for polymerization to produce aromatic polycarbonates (see Patent Documents 26: Japanese Patent Application Laid-Open No. 9-255772 (corresponding to European Patent No. 0892001, and U.S. Pat. No. 5,747,609)).
However, in all of these prior art documents in which the production of aromatic carbonates using the reactive distillation method is proposed, there is no disclosure whatsoever of a specific process or apparatus enabling mass production on an industrial scale (e.g. a more than 1 ton/hr), nor is there any description suggesting such a process or apparatus. For example, for the case of producing mainly methyl phenyl carbonate (MPC) from dimethyl carbonate and phenol, the maximum amount produced of methyl phenyl carbonate (MPC) disclosed in documents hitherto has been that disclosed in Patent Documents 24 and 25, but this amount is less than 10 kg/hr, which is not an amount produced on an industrial scale.
On the other hand, several proposals have also been made regarding processes for separating out the alcohols from a reaction mixture containing methanol by-produced through a transesterification reaction between dimethyl carbonate and phenol. For example, there have been proposed a process in which the above reaction is carried out using a reactor in which a liquid phase portion is divided into a plurality of reaction sections, and the reaction liquid passes through the sections in order and flows out from the reactor, and gas is withdrawn from a vapor phase portion, and subjected to heat exchange, before being subjected to separation in a distillation column (see Patent Document 27: Japanese Patent Application Laid-Open No. 2003-113144), and a process in which the same reactor is used, and the gas is withdrawn from the vapor phase portion, and subjected to heat exchange and thus liquefied, before being subjected to separation by distillation at a higher pressure than that of the pressure in the vapor phase portion of the reactor (see Patent Document 28: Japanese Patent Application Laid-Open No. 2003-155264). However, the object of these processes is to separate out the gas components with reduced energy consumption when carrying out reaction using a continuous stirring tank reactor as mentioned earlier as the reactor. Moreover, the composition of the gas components obtained using this reaction system is, for example, 98.1% by weight of dimethyl carbonate, 1.4% by weight of methanol, and 0.5% by weight of phenol, methyl phenyl carbonate and so on (See Patent Document 28), which differs greatly to the composition of a low boiling point reaction mixture containing a by-produced alcohol produced using a reactive distillation system. This is obviously different therefore to the case of separating out to a prescribed concentration the alcohol from a low boiling point reaction mixture produced using a reactive distillation system. A process has also been proposed in which a liquid containing approximately 10 to 74% by weight of methanol is distilled off at approximately 30 g/hr from the top of a distillation column provided on top of a tank reactor (see Patent Document 29: Japanese Patent Application Laid-Open No. 6-157410). However, the above patent documents either relate to processes carried out on a small amount laboratory scale, or else merely relate to carrying out a comparative calculation of the amount of energy required for the distillation. There is no specific description or suggestion whatsoever in any of the above patent documents relating to carrying out separation on an industrial scale.
In the case of carrying out transesterification reaction between the dialkyl carbonate and the aromatic hydroxy compound using the reactive distillation system, the by-produced alcohols are generally continuously withdrawn from an upper portion of the reactive distillation column as a low boiling point reaction mixture also containing compounds present in the reaction system having a lower boiling point than the at least one aromatic carbonate produced, for example the dialkyl carbonate and aromatic hydroxy compound constituting the starting material, a by-produced alkyl aryl ether and so on. Since this transesterification reaction is an equilibrium reaction having an extremely low equilibrium constant and the by-produced alcohols impede the reaction, in the case of industrial implementation, it thus becomes important to continuously carry out separation and recovery from the low boiling point reaction mixture into a component having a low content of the alcohol and a component containing mainly the alcohol efficiently and stably for a prolonged period of time.
Several proposals have been made regarding processes for carrying out separation by distillation of the low boiling point reaction mixture containing methanol and dimethyl carbonate withdrawn from an upper portion of a column when subjecting dimethyl carbonate and phenol to transesterification reaction using the reactive distillation system. These are an extractive distillation process in which the methanol is withdrawn from the top of the column while extracting the dimethyl carbonate using dimethyl oxalate as an extractant (see Patent Document 30: Japanese Patent Application Laid-Open No. 7-101908), an extractive distillation process in which the methanol is withdrawn from the top of the column while extracting the dimethyl carbonate using ethylene carbonate as an extractant (see Patent Document 14), a process in which distillation is carried out at normal pressure and a mixture containing approximately 70% by weight of methanol and approximately 30% by weight of dimethyl carbonate is obtained from the top of the column (see Patent Documents 19), a process in which a mixture containing 64.5% by weight of methanol and 35.5% by weight of dimethyl carbonate is obtained from the top of the column (see Patent Document 20), and a process in which a mixture containing 60 to 40% by weight of methanol and 40 to 60% by weight of dimethyl carbonate is obtained from the top of the column (see Patent Document 15).
However, with the extractive distillation processes, a large amount of the extractant must be used, and after the extraction, the extractant and the dimethyl carbonate must be further separated from one another. With the other processes, the content of the methanol in the liquid withdrawn from the top of the column is low at less than 80% by weight, and hence this is undesirable in the case of use as a raw material for producing dimethyl carbonate. Moreover, in the separation/recovery processes described in the above patent documents, the amount of methanol separated out and recovered is not more than a few hundreds of grams per hour. Even in Patent Document 20, which describes the largest amount, the amount of methanol processed is only approximately 0.9 kg/hr.
Furthermore, regarding the time for which the separating out of the methanol by distillation is carried out continuously in the above patent documents, the longest time in the case of Patent Document 29, in which the reaction system other than the reactive distillation system is used, is at most 720 hours. The longest time in the case that the reactive distillation system is used is only 2 weeks (Patent Document 19), with the others being 10 days (Patent Document 15), or the time taken for a steady state to be attained (Patent Documents 14 and 30). These periods of time are extremely short, and there is no disclosure or suggestion of an industrial separation process in which the distillation operation is carried out stably for a prolonged period of time of several thousand hours, for example 5000 hours.
In this way, for the case of industrially producing aromatic carbonates using the reactive distillation method, there has been no specific disclosure or suggestion whatsoever regarding an industrial process or apparatus for separating out a large amount of a by-produced alcohol efficiently and stably for a prolonged period of time.